Insulated underground power cables are extensively used to transport electric energy from power generating plants or substations to load centers where the energy is required or to interconnect various electrical equipment in a plant or facility. Power cables used at elevated voltages, for example three thousand volts or more, basically consist of a central conductor surrounded by a thin concentric layer of a semi-conducting material which is referred to as a conductor shield. The conducting shield, in turn, is surrounded by a concentric layer of insulating material such as oil impregnated paper, polyethylene, or other polymers. This insulating material is covered by a second concentric semi-conducting layer referred to as an insulation shield over which a metal conductor is applied in the form of a thin helically wound concentric cylindrical wires or flat strips which provide a ground or neutral. This assembly may be covered by a thin electrically insulating jacket. Power cables designed to operate below three thousand volts and communication cables may not have semi-conducting shields and neutrals.
Insulated cables are intended to operate safely and effectively over lifespans exceeding thirty years. However, because of manufacturing defects, installation errors, or just normal aging under adverse conditions, cables may either develop incipient faults or completely fail to support the electric voltages imposed thereon. Incipient faults often manifest themselves by a phenomenon called "partial discharge" (PD). Because of a defect developing within or adjacent to the insulation of a cable, intermittent arcing (partial discharge) occurs within the insulation. As used herein, the term "partial discharge" refers to a fault within the cable which will not cause immediate failure but which may lead to eventual failure. The energies involved in this process are extremely small, and depending upon the type of insulating material, a more or less rapid localized deterioration of he material occurs. Eventually, incipient faults lead to a complete breakdown of the cable. Should this occur during a critical period, such as during a peak load for a utility or a critical manufacturing process for an industrial customer, the repercussions in terms of financial losses and customer inconveniences can be quite severe.
If users could monitor the condition of the cables and be able to pinpoint the location of defects and the severity of the defects, such users would be able to schedule non-use of the electric power during non-critical periods to effect the necessary repairs and avoid catastrophic breakdowns.
Assuming that a total cable failure occurs, it is an absolute necessity to locate the site of the fault as promptly as possible in order to perform rapid repairs. The methods used to locate cable faults need to be non-destructive, that is, they should not impose excessive stresses on the portions of cables which are not at fault, as this may create undue deterioration of the cable insulation. At the present time, no commercial instrumentation is known to be available which can locate and characterize cable incipient defects or faults in a non-destructive way in field installations. Some instruments exist which are able to detect the existence of a defect but not its location. Other instruments have been reported to be able to locate a fault in a cable while it is still in the factory, or in a well-shielded room. However, to use these known instruments, both cable ends must be accessible simultaneously, and the measurements will contain some ambiguity as to which end of the cable is taken as a reference to measure the location of the defect.
The location of faults in power lines has been addressed in the prior patent art. U.S. Pat. No. 2,628,267 measures the time of arrival of surge currents and reflections thereof emanating from a ground at a site in the cable. U.S. Pat. Nos. 3,244,975 and 3,255,406 disclose what is referred to as an impulse-reflection method of testing cables where the time difference of detection of transmitted and reflected pulses are measured to determine fault location. U.S. Pat. No. 4,104,582 measures the time by clocking a counter. U.S. Pat. No. 4,491,782 discloses a technique of recording impulse reflection where impulse reflection signals are measured during normal operation and also under fault conditions and makes a comparison after fault occurs to determine the location of the fault in a cable.
The known prior patent art appears to address the determination of only large faults which already will cause or have caused failure, and of incipient faults, which are sometimes referred to as partial discharges, in order to determine the remaining time of useful service of the cable.
Accordingly, the present invention provides a new and improved method and instrumentation for testing electrical power cables to determine the location of incipient faults therein and to determine the potential useful remaining life of the cable.